Add parameters smtp_bind_address, smtp_bind_address6 and smtp_helo_name

[webcompas]

The configuration parameters smtp_bind_address, smtp_bind_address6 and smtp_helo_name can actually only be set using extra_main_parameters. So support for these should be added using the following default values:

smtp_bind_address =
smtp_bind_address6 =
smtp_helo_name = $myhostname

[thias]

Clean pull request welcome! Look at the existing parameters, keep the relevant order and make sure that by default no configuration files change (not even empty lines).

[webcompas]

I just did the changes and created a pull request (#83).

[HielkeJ]

thias,

Is it possible to merge this request?

Thanks!